Circuit Schematic learn

This device is designed to be a simple, inexpensive ‘comparator’, intended for use in a solar cell power supply setup where a quick ‘too low’ or ‘just right’ voltage indicator is needed. The circuit consists only of one 5V regulator, two transistors, two LEDs, five resistors, two capacitors, and one small battery. Although a 4-V battery is indicated, 4.5 V (3 alkalines in series) or 3.6 V (3 NiCd cells in series) will also work.

The specifications of voltage regulator IC1 are mainly determined by the size and number of the solar cells and the current pull of the equipment connected to the output. Here the low-drop 4805 is suggested but other regulators may work equally well as long as you observe the output voltage of the solar cells. Transistors T1 and T2 are complementary types i.e. one each of the pnp and npn variety.

Circuit diagram:

Although the ubiquitous BC557B (pnp) and BC547B (npn) are indicated, any small-signal equivalents out of the junk box will probably do. The values of voltage dividers R1/R6 and R3/R4 may need to be adjusted according to the type of transistor and its gain, or according to the desired voltage thresholds. Using the resistor values shown in the schematic, LED D2 turns on fully when the voltage is just above 5 volts.

LED D1 turns on when the voltage drops below 4.2 volts or so. Between those two thresholds, there is a sort of no man’s land where both LEDs are on dimly. A buzzer or other warning device could be connected across the terminals of LED D1 to give a more substantial warning if the voltage drops below operating limits. The current consumption of the circuit is about 20 mA at 5 V, and it decreases with the voltage supplied by the solar cells.

By combining a common 78L05 with an integrated audio amplifier of the type TDA2030, an adjustable voltage regulator can be constructed in a very simple manner that works very well. The output voltage is adjustable up to 20 V, with a maximum current of 3 A. Since the TDA2030 comes complete with a good thermal and short-circuit protection circuit, this adjustable regulator is also very robust. As illustrated by the schematic, the design of this circuit is characterized by simplicity that is hard to beat. In addition to the two ICs, the regulator contains actually only two potentiometers and a few capacitors.

The adjustment is done by first turning potentiometer P1 to maximum (wiper to the side of the 78L05) and subsequently turning trimpot P2 until the desired maximum output voltage is reached. P1 is then used to provide a continuously adjustable voltage between this maximum and nearly zero volts. At relatively small output currents there are no specific requirements regarding the cooling. However, when the output current exceeds 1 A, or if the input to output voltage difference is quite large, the amplifier IC has to dissipate too much power and a small heatsink is certainly appropriate.

Using this circuit, you can change the speed of the fan from your couch or bed. Infrared receiver module TSOP1738 is used to receive the infrared signal transmitted by remote control. The circuit is powered by regulated 9V. The AC mains is stepped down by transformer X1 to deliver a secondary output of 12V-0-12V. The transformer output is rectified by full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C9 and regulated by 7809 regulator to provide 9V regulated output. Any button on the remote can be used for controlling the speed of the fan. Pulses from the IR receiver module are applied as a trigger signal to timer NE555 (IC1) via LED1 and resistor R4. IC1 is wired as a monostable multivibrator to delay the clock given to decade counter-cum-driver IC CD4017 (IC2).

Out of the ten outputs of decade counter IC2 (Q0 through Q9), only five (Q0 through Q4) are used to control the fan. Q5 output is not used, while Q6 output is used to reset the counter. Another NE555 timer (IC3) is also wired as a monostable multivibrator. Combination of one of the resistors R5 through R9 and capacitor C5 controls the pulse width. The output from IC CD4017 (IC2) is applied to resistors R5 through R9. If Q0 is high capacitor C5 is charged through resistor R5, if Q1 is high capacitor C5 is charged through resistor R6, and so on. Optocoupler MCT2E (IC5) is wired as a zero-crossing detector that supplies trigger pulses to monostable multivibrator IC3 during zero crossing. Opto-isolator MOC3021 (IC4) drives triac BT136.

Resistor R13 (47-ohm) and capacitor C7 (0.01µF) combination is used as snubber network for triac1 (BT136). As the width of the pulse decreases, firing angle of the triac increases and speed of the fan also increases. Thus the speed of the fan increases when we press any button on the remote control. Assemble the circuit on a general-purpose PCB and house it in a small case such that the infrared sensor can easily receive the signal from the remote transmitter.